Bazel client, JNI, Windows: impl. CreateJunction

Implement a CreateJunction function in the Windows
JNI library. Also move a bit of code from
file_windows to the JNI library, where it is
(also) needed.

This implementation is an improved version of
`blaze_util::SymlinkDirectories` in
blaze_util_windows: this version handles Windows
paths as `name` and `target`, and performs more
validation (e.g. on the length of `target`), plus
has more comments explaining the logic. In a
subsequent change I'll start using this new
function in blaze_util_windows.

This method will also be helpful in tests: we will
no longer have to shell out to mklink.

See https://github.com/bazelbuild/bazel/issues/2107

--
Change-Id: I7e9b085fdc2ba47be83da5319bded02bd323e71b
Reviewed-on: https://cr.bazel.build/8892
PiperOrigin-RevId: 147585207
MOS_MIGRATED_REVID=147585207

1 files changed, 1 insertions, 6 deletions

diff --git a/src/main/cpp/util/file_windows.cc b/src/main/cpp/util/file_windows.cc
index 9d22444dc8..e2e12950eb 100644
--- a/src/main/cpp/util/file_windows.cc
+++ b/src/main/cpp/util/file_windows.cc
@@ -35,6 +35,7 @@ using std::pair;
 using std::string;
 using std::unique_ptr;
 using std::wstring;
+using windows_util::HasUncPrefix;
 
 // Returns the current working directory as a Windows path.
 // The result may have a UNC prefix.
@@ -101,12 +102,6 @@ static bool HasDriveSpecifierPrefix(const char_type* ch) {
   return CharTraits<char_type>::IsAlpha(ch[0]) && ch[1] == ':';
 }
 
-template <typename char_type>
-static bool HasUncPrefix(const char_type* path) {
-  return path[0] == '\\' && (path[1] == '\\' || path[1] == '?') &&
-         (path[2] == '.' || path[2] == '?') && path[3] == '\\';
-}
-
 static void AddUncPrefixMaybe(wstring* path) {
   if (path->size() >= MAX_PATH && !HasUncPrefix(path->c_str())) {
     *path = wstring(L"\\\\?\\") + *path;